The present invention relates to marking devices and is concerned in particular with an apparatus that produces marking bands along insulated wires, cables, conduits and the like.
In the production of a multi-conductor cable, composed of individually insulated wires, it is usually necessary to provide a way of distinguishing each conductor from all of the others to facilitate rapid and accurate connections when the cable is installed. Many means of accomplishing this end have been used, including the use of insulating compounds of different colors, marking devices which stripe or periodically band each insulated wire with inks of contrasting colors, or other marking devices which periodically print a series of numbers and letters on the insulation, again in contrasting colors.
Using different color insulations is simplest, but limits the number of readily distinguishable conductors in a cable. For cables with large numbers of conductors, several of the marking schemes must be used in combination. Most marking methods require the conductor to come into contact with a print wheel and use slow-drying inks. This process limits wire line speeds to under a few hundred feet per minute and is therefore expensive.
A newer process in use for several years allows closely spaced color bands to be applied to insulated wire moving at high extrusion speeds. This technique employs an ink jet dispenser which is rapidly oscillated alternately above and below the axis of the insulated wire as it emerges from the extruder head. A semicircular segment of a band is applied to the insulation each time the ink jet crosses the wire. Since the insulating compound at this point is not yet cooled below its melting point, a fast-drying ink which amalgamates with the compound can be used. Drying and amalgamation are complete before the product enters a water cooling trough which is located downstream of the extruder in the wire production line.
Since the process just described only covers 180 degrees of the wire circumference, a second jet dispenser oscillating in synchronism with the first is directed at the opposite side of the wire to complete the band. The ink dispensers cannot be located directly opposite each other, since the ink jets would interfere with one another. One dispenser is therefore displaced longitudinally along the wire line by a multiple of the spaces between the bands. A mechanical adjustment of the dispensers is customarily provided to allow this displacement to be varied by a small amount which brings the two band segments into exact alignment.
Each ink jet dispenser is oscillated through an angle of approximately 20 degrees or so by a transducer comprised of a permanent magnet cyclically driven in the alternating field of an electromagnet. The permanent magnet is resiliently restrained by a torsion spring to oscillate with the alternating field, and the mechanically resonant frequency of the moving assembly is set substantially higher than the highest required driving frequency. The rotational oscillations of the magnet are conveyed to an ink jet nozzle by means of a shaft which is supported in a bearing. The permanent magnet is supported within the driving electromagnetic structure by means of this bearing and by the torsion spring. The design of the transducer is such that its instantaneous angle of rotation is directly proportional to the instantaneous magnitude of the alternating current in the electromagnet coil.
The alternating driving current for the electromagnet is obtained from an amplifier, which in turn is driven by an AC tachometer rotated by the moving wire. Since the tachometer output frequency is proportional to the wire speed, the frequency of oscillation of the ink nozzles is also directly proportional to wire speed. Thus, the average spacing of the bands is independent of wire speed, and depends only on the constant of proportionality between wire speed and tachometer output frequency.
A number of adjustments are necessary to obtain satisfactory operation of the prior art system described above. The midpoint of the oscillation must coincide with the wire axis to avoid alternate narrow and wide spacing of the bands. Adjustment of this midpoint can be achieved by superimposing a direct current on the alternating current for each transducer to produce the required average deflection.
The distance between the oscillating jets is varied by mechanical means to bring the two half segments of each band into coincidence.
The gain of the system amplifiers must be set so as not to distort the alternating driving signals. Even order distortion in particular must be avoided, since this shifts the zero offset as amplitude changes. The voltage output from the tachometer is linearly related to frequency. The impedance of the electromagnetic transducer, however, is not a simple function of frequency. As a result, it is necessary to manually adjust amplifier gains as line speed varies.
In designs of the system now available, all the various adjustments must be made in accordance with visual observation of the ink jets under a stroboscope. The two ink jets require an amplifier gain adjustment, two amplitude adjustments, two centering adjustments, and a mechanical alignment to match segments of each marking band, all by stroboscopic observation. Since the burden of these adjustments falls upon the extruder operator who may not fully comprehend which adjustment to make for a given deviation from normal marking, the result is usually marking of poor and variable quality.
It is accordingly a general object of the present invention to provide an improved band marking controller which eliminates the possibility of distorting the electrical signal in the amplifier, eliminates the mechanical band segment alignment adjustment after initial setting, reduces the need for frequent adjustment as wire line speed changes, and provides separate indicators of amplitude and centering for each ink jet. The improved controller allows a relatively unskilled operator to consistently produce properly banded wire.